This invention relates to liquid metal cooled fast breeder nuclear reactor constructions.
In one known construction of liquid metal cooled fast breeder nuclear reactor the reactor fuel assembly is submerged in a pool of liquid metal coolant in a primary vessel which is housed in a concrete vault. The fuel assembly is carried by a strongback and is surrounded by an inner tank defining an inner or hot region of the pool and an outer or cool region of the pool. There are coolant pumps in the outer region which circulate coolant upwardly through the fuel assembly by way of the strongback thence to heat exchangers in the hot region which discharge to the cool region. During operation of the reactor the temperature of the hot region is approximately 540.degree. C. and that of the cool region approximately 370.degree. C. and there is a differential in the coolant levels in the regions due to the differential pressure over the inlet and outlet ports of the pump. In order to reduce the complex stresses in the wall of the inner tank due to the temperature differential across it and to reduce temperature degradation of the hot region coolant, the inner surface of the wall is clad with thermal insulation. Several forms of thermal insulation have been used or proposed, but for various reasons have proved unsatisfactory. One form of insulation comprises spaced sheets of stainless steel defining a radial series of compartments in which liquid coolant will stagnate, each sheet comprising a pair of membranes secured together face-to-face and welded together in quilted pattern manner but there is difficulty in the pre-operation water testing of this material and, in use, superficial thermal expansion sets up complex stresses in the material. Another form of insulation comprises a layer of stainless steel blocks attached to the wall surface and covered with a stainless steel membrane but the attachments required for the blocks and membrane are so numerous that the insulation becomes very expensive.
In a co-pending United States patent application entitled Liquid Metal Cooled Fast Breeder Nuclear Reactor Construction by John Graham Durston and John Richard Hind filed on the same day as the present application there is disclosed a liquid metal cooled fast breeder nuclear reactor construction comprising a nuclear reactor fuel assembly submerged in a pool of coolant within a primary vessel and having an inner tank surrounding the fuel assembly and defining inner and outer regions of the pool, a coolant pump in the outer region for circulating coolant through the fuel assembly and through a heat exchanger disposed in the inner region, wherein the inner tank has a vertically extending series of spaced fins attached to its inner surface, the fins extending continuously around the inner surface and being inclined upwardly to form, in cooperation with the inner wall surface of the inner tank, a series of coolant retaining troughs, the free ends of the fins being disposed above the roots of the adjacent upper fins.
In operation of the nuclear reactor the coolant contained by the troughs remains substantially static the denser cooler liquid metal falling in the troughs to the inner wall surface of the inner tank thereby cladding substantially the whole of the submerged surface with cooler coolant.